A method and a device of this kind may be used for measuring the thickness of one or several layers within different areas, and the only condition is that at least one of the layers or the substrate is a conductor of electricity, so that the electromagnetic alternating field will generate eddy currents when penetrating through that layer or the substrate. It should be noted that when reference is made to a "conducting layer" this phrase is intended to include within its scope the substrate. To illuminate, but not in any way to restrict the scope of the invention, the particular use of such a method and such a device for measuring the thickness of layers created on substrates constituted by fuel rods in nuclear power reactors will be discussed hereinafter, although this is not in any way to be interpreted as limiting the scope of the invention.
Such reactor fuel rods used in nuclear power reactors are usually made of a zirconium alloy (Zircalloy) and contain the fuel itself, usually in the form of a number of small sintered pellets of uranium dioxide. In the very reactive environment surrounding these fuel rods, various chemical reactions will take place, of which one is the formation of a layer of zirconium dioxide outside the zirconium alloy itself. This layer grows inwardly so that the thickness of the substrate will gradually be reduced at the same time that the thickness of the oxide layer increases. A so called "crud" layer is also produced outside the oxide layer, and this consists usually of a mixture of Fe, Zn and O. It is extremely important that the development of the thickness of the oxide layer be under control since this may, primarily for security reasons, not exceed a determined value. If this is the case, the fuel rod has to be taken out of the reactor and a new fuel rod introduced.
Due to the fact that the substrate in this case is a conductor of electricity and since it consists of the above-mentioned zirconium alloy, the method according to the introduction may be used for determining the thickness of the oxide layer. When doing so, it has heretofore been that a coil generates an electromagnetic alternating field with a certain frequency close to layers having different thicknesses at the same time as any quantity of the alternating field is measured, such as the current flowing through the coil at a predetermined voltage thereacross, for calibration of the measuring coil. The eddy currents generated by the alternating field in the conducting substrate will disturb the alternating field itself, and this disturbance is reduced with increasing distance between the coil and the substrate, i.e., with the thickness of the oxide layer and the crud layer possibly located thereupon. However, it has turned out that the results obtained through these measurements have been confusing, since it seems as if layer thicknesses which are much too large have been indicated. It could be established by studying the fuel rods in question through different methods that, in such cases, the measurement has often grossly overestimated the layer thicknesses being measured.
Thus, the use of this known measuring method means that fuel rods will be taken away and changed unnecessarily early with high additional costs as a consequence, and it will at the same time, not be possible to really rely on the obtained measurement results and it will not be possible to carry out any reliable studies of the oxide formation rate and how this is developed for different types of zirconium alloys.
It may also be mentioned that grossly overestimated values of the oxide thicknesses have been produced, especially in reactors in which zinc has been measured in so as to reduce the radiation doses in circulation and auxiliary systems.